1919.] Radiation Pressure. 301 
After the movement of the reflector to S’, the pressures 
are DH and EH of the same magnitude as before, because 
SS’ is short. Let DF be the pressure-volume line for gas now 
in SS’, F being on the line MH. Then F gives us the first 
position, C, of the impervious screen. This screen, in com- 
pressing the gas SC to SS’, does work represented by t the area 
DFGH. The gas in A does work on it, represented by 
EFGH. Thus the work done by the reflector in the dis- 
placement SS’ is positive, and is represented by the area 
DFE. The work which would be equal to the product of the 
pressure on the reflector, and its pe BeBe, is represented 
by the area LDEM and is in this case negativ 
The work done by the anes when the pe in A is com- 
pressed, can be found simi 
It is thus shown that ‘ks SO end rate of working, 
is not given by the instantaneous pressure on the reflector, 
and its velocity. Hence the average rate at which a moving 
s s’ c 
.D 
c 
M E \F 
H G Y 
Fig. 2. vs 
X 
reflector does work ke not a direct measure of the pressure on 
a stationary reflecto 
his pidialastosee n be applied to strains in any material 
media. The following i illustration -— make it sieecets An 
In the case of an electric field, this treatment must be 
cidiae Let there be an electric field in A, with displace- 
